1. Field of the Invention
The invention relates to a hydraulic control apparatus for performing shift control in an automatic transmission provided in a vehicle or the like. More particularly, the invention relates to a hydraulic control apparatus and a hydraulic control method for an automatic transmission, which suppresses shift shock when a shift operation is performed in the transmission.
2. Description of the Related Art
One type of known transmission that transmits torque and rotation speed generated by the engine to the driving wheels appropriately according to the running state of the vehicle is an automatic transmission which automatically establishes the optimum gear ratio between the engine and the driving wheels.
Two examples of such known automatic transmissions that are used in vehicles are planetary gear type transmissions that establish a gear (hereinafter also referred to as “speed”) using a planetary gear set together with clutches and brakes, and belt-type continuously variable transmissions (CVT) that adjust the gear ratio continuously (i.e., in a stepless manner).
In a vehicle provided with a planetary gear type automatic transmission, a shift map having shift lines (i.e., gear shift lines) for establishing the optimum gear according to vehicle speed and accelerator operation amount (or throttle opening amount) is stored in an ECU (Electronic Control Unit) or the like. The ECU calculates a target gear based on the vehicle speed and the accelerator operation amount referencing the shift map, and automatically shifts gears (i.e., speeds) by selectively applying and releasing the clutches and brakes, which are friction apply elements, in a predetermined combination to establish that target gear.
Also, a vehicle provided with this kind of automatic transmission has a shift lever which is operated by a driver. Operating this shift lever enables the automatic transmission to be switched between various ranges or modes, such as a P-range (parking range), an R-range (reverse running range), an N-range (a neutral range), and a D-range (forward running range or drive range). Moreover, in recent years, automatic transmissions with a manual shift function (so-called automatic transmissions with a sequential mode or sequential mode automatic transmissions) have also come into practical use. These transmissions also enable the driver to change gears in the transmission at will by operating the shift lever.
Also, among these kinds of transmissions there are some which perform a so-called coast downshift in which the transmission downshifts according to a preset shift condition with the vehicle speed as a parameter when decelerating while the accelerator is off. The shift control apparatus described in Japanese Patent No. 2917601 is one such example. This shift control apparatus shortens the shift time by performing a “skip shift” in which it downshifts by skipping over a gear or gears when a multiple gear downshift (i.e., a downshift through multiple gears) is necessary, i.e., when a shift into a gear that is two or more gears lower than the current gear (i.e., into a gear with a larger gear ratio) is necessary, when the vehicle is decelerating.
However, depending on the type of downshift, when this skip shift is performed, the release and apply timing of the friction apply elements (i.e., clutches and brakes) of the automatic transmission may be off (i.e., an apply operation or the like may end up being performed before the torque capacity is obtained in the apply-side friction apply element), and as a result, the engine may race and there may be a loss of torque, and shift shock may result.
To avoid this problem, it is desirable to perform a multiple downshift operation, i.e., a downshift operation through multiple gears, by repeatedly downshifting one gear at a time (i.e., an operation referred to as “sequential shifting”) as described in Japanese Patent Application Publication No. 2005-344773 (JP-A-2005-344773).
However, in transmissions that perform this kind of sequential shifting, there is a possibility that the gear suitable for the vehicle speed may not be able to be established in cases such as when the vehicle is decelerating relatively quickly, for example, because the total shift time increases.
More specifically, the shift timing of the downshift in a power-off state (i.e., when the engine is being driven by the driving wheels) is typically set with only the vehicle speed as the parameter, as described above.
Therefore, for example, when a shift signal for a shift into 4th gear has been output based on a shift map but the vehicle has already decelerated below the vehicle speed corresponding to 4th gear in the transmission, the 4→3 shift line on the shift map is crossed such that a shift signal for a shift into 3rd gear is output. At this time, a shift operation is started by the clutches and brakes starting to be released and applied (i.e., by the start of a clutch-to-clutch shift operation) so that 3rd gear will be established in the transmission.
In this case, if the vehicle continues to decelerate relatively rapidly, the 3→2 shift line on the shift map will be crossed before the shift operation into 3rd gear is complete (i.e., while the clutches and brakes are in the middle of being released and applied) such that a shift command for a shift into 2nd gear will be output. Incidentally, because the shift operation into 3rd gear in the transmission is not yet complete, the shift operation into 2nd gear (i.e., the release and apply operations of the clutches and brakes to establish 2nd gear) must wait to be performed until the shift operation into 3rd gear is complete. That is, a shift operation is performed into a higher gear (i.e., a gear with a lower gear ratio) than the gear that is appropriate for the vehicle speed.
In such a situation, the turbine speed (i.e., the input rotation speed of the transmission) is low as a result of the falling vehicle speed so an increased amount of torque multiplication is being produced by the torque converter. During the shift operation into 2nd gear after the shift operation into 3rd gear is complete, the turbine speed increases (races), and it is at this time that the apply operation of the clutches and brakes to establish 2nd gear would be performed. As a result, a phenomenon similar to that of a so-called power-on downshift may occur which produces shift shock that pulls the vehicle forward. In this way, the sequential shifting of the related art leads to a delay of a shift operation into the appropriate gear, as well as a deterioration in drivability from shift shock. These kinds of problems become more evident as the number of gears (i.e., speeds) in automatic transmissions increases. In particular, automatic transmissions in recent years are being made to have more and more gears (i.e., speeds), which makes them increasingly susceptible to these kinds of problems.
Even if an automatic transmission shifts into a lower gear in response to an operation of the shift lever while the vehicle is decelerating (i.e., even if there is a downshift operation in the sequential mode), the foregoing problems when the gear appropriate for the vehicle speed is unable to be established may still occur, just as described above.
Incidentally, the problems with a transmission that performs sequential shifting are not limited to a case in which a shift command to shift into the next gear is output while a shift operation such as that described above is being performed. That is, those problems may also occur with the shift timing (i.e., the timing at which there is a shift demand when a shift operation is not being performed) when the vehicle is in the power-off state (i.e., when the engine is being driven by the driving wheels) and the vehicle is decelerating relatively rapidly.